1. Field of the Invention
This invention relates to the preparation of precipitated calcium carbonate (PCC) compositions and the resulting PCC composition products. More particularly, the invention relates to the preparation of precipitated calcium carbonate (PCC) compositions having reduced high-shear viscosity in aqueous slurry form, and containing PCC particles of narrower particle size distribution, while reducing requirements for relatively cost-intensive preparation steps such as evaporative-based dehydration procedures. The precipitated calcium carbonate composition products prepared by the invention are especially useful as a paper coating pigment and filler, among other things.
2. Description of the Related Art
Precipitated calcium carbonate is an extremely versatile coating aid, filler and pigment that is employed in a wide variety of commercial products including paper, paint, plastics, rubber, textiles, and printing inks. Precipitated calcium carbonate (PCC) is used on a large scale in paper filling and coating applications in particular. PCC is utilized to increase the opacity and brightness of paper, among other things. In addition to enhancing the opacifying and brightening characteristics of paper, PCC also imparts a high resistance to yellowing and aging of the paper. PCC offers cost advantages over other conventional additives used for increasing the opacity and brightness of paper, such as calcined clay or titanium dioxide.
As a practical matter, many contemporary paper coating machines operate at high running velocities, in which the paper coating composition is applied to a side of running base paper stock using a blade coater. As a consequence, the paper coating composition encounters high shearing forces during such blade coating, making it desirable to have a paper coating composition that displays low viscosity at high shear conditions so that it is sufficiently flowable to form a thin uniform coating when applied to paper base stock. An impediment to achieving such high-speed flowability is that paper coating compositions must employ a relatively high solids content in order to hold down the coating drying requirements, which tends to generally increase the viscosity. For instance, paper coating compositions comprised of aqueous slurries containing PCC have about 70 wt % or higher total solids content. However, once coated upon base paper, a paper coating composition needs to be relatively immobile so that it does not run or wick, to preserve the uniformly applied coating. Thus, it is also desirable that the paper coating composition be able to assume a relatively higher viscosity behavior immediately upon emerging from the high shear conditions associated with the blade coating. Thus, a challenge has existed to impart a stable, pseudoplastic-like rheological character in a paper coating slurry composition, as opposed to dilatant behavior, as those terms are customarily defined for non-Newtonian fluids. Namely, the apparent viscosity of a pseudo-plastic fluid material decreases with increasing shear rate, which is opposite to rheological behavior of a dilatant fluid material which has an apparent viscosity that increases with increasing shear rate. Moreover, the pursuit of the desired pseudo-plastic like rheological character for paper coating compositions cannot come at the expense of the light scattering properties required of the PCC. Ideally, enhancements to the optical performance properties of the PCC somehow would be provided together with the desired rheological improvements all in a single PCC containing composition.
The prior art has proposed manipulating the preparation procedures used to make PCC slurries in efforts to improve the ultimate properties of the resulting PCC composition used in paper coating applications. For example, U.S. Pat. No. 5,833,747 to Bleakley et al. describes a process scheme for making paper coating pigments in which a slurry of PCC particle aggregates made by carbonation of milk of lime is subjected to the steps of partial dewatering using a tube pressure filter in conjunction with a separate comminution step using a single pass attrition grinding mill fitted with a 250 horsepower (186 kW) motor and containing silica sand as grinding media such that at least 100 kW-hr of energy per dry ton of PCC is dissipated in the PCC suspension (where the sequence of the partial dewatering and comminution steps optionally can be switched). Bleakley et al. describe a PCC product provided having a particle size distribution such that 70% of the particles have an equivalent spherical diameter of less one micrometer and at least 50 percent of the particles have an equivalent spherical diameter of less than 0.5 micrometers.
U.S. Pat. No. 5,879,442 to Nishiguchi et al. teaches a method for making an aqueous slurry of PCC suitable for paper coating compositions in which a PCC slurry is subjected to a primary dispersion in the presence of a viscosity reducing agent using a mixer such as xe2x80x9cCowlesxe2x80x9d type mixers and agitating dispersers; and then dry ground calcium carbonate is added and the resulting slurry subjected to mixing again with xe2x80x9cCowlesxe2x80x9d type mixers and agitating dispersers as a secondary dispersing treatment; followed by a tertiary dispersing treatment in which the calcium carbonate slurry is subjected to a sand grinding treatment, preferably using 2-4 runs of the sand grinding treatment and also with admixture of a viscosity-reducing agent. The method described in U.S. Pat. No. 5,879,442 is used to provide an aqueous slurry of calcium carbonate particles (70 to 85 total wt % calcium carbonate) having a median particle size of 0.2 to 2.0 xcexcm and a specific surface area of ranging from 5 to 25 m2/g, and the slurry has a viscosity not exceeding 1000 cP at 25xc2x0 C. and a high shear viscosity not exceeding 200 cP at 25xc2x0 C.
A need has existed for development of even higher quality aqueous slurries of PCC suitable for paper coating applications which can be made in a cost effective manner.
The present invention achieves the above and other objectives by providing a unique approach to preparing PCC compositions such that the resulting PCC compositions are endowed with reduced high-shear viscosity in aqueous slurry form at high solids content and contain PCC particles of narrower particle size distribution. As another aspect of the invention, the additional cost otherwise associated with the use of evaporative-based dehydration procedures to increase the PCC solids content in a PCC slurry or associated with the adding of dry CaCO3 pigment to build slurry solids can be reduced or even eliminated by a facile inventive modification made in a decanting operation within the process flow according to this invention. The present invention is particularly well-suited for providing PCC slurries useful for paper coatings applications including medium and low coating weight, woodfree paper coatings, although it has wider applicability and is by no means limited thereto.
As a general embodiment of the invention, there is a process for the preparation of a precipitated calcium carbonate (PCC) composition, comprising the step of wet media mill grinding an aqueous slurry of PCC made by carbonation of lime milk, with the grinding being done at a low solids content in a plurality of separate grinding stages while essentially free of the presence of organic dispersant. After the multi-stage wet grinding step, the ground PCC slurry is partially dewatered in the presence of an organic dispersant in a solid bowl centrifuge. Then, the partially dewatered PCC slurry is subjected, in the presence of an organic dispersant, to fluid shearing forces created in a rotor/stator type mixer. While these three steps are advantageously used together within a single PCC process flow for optimal effect, the individual steps also can be incorporated singly or in lesser combinations thereof into different PCC manufacturing schemes to beneficial effect.
In one aspect of the invention, PCC particles in a crude, synthesized PCC slurry of relatively low solids content (e.g., 15-25 wt % PCC) are subjected to multi-stage wet media grinding (including a multi-pass mode through a single mill or conducting the slurry through a series of separate mills). It has been discovered that it is possible to mill the PCC particles in this manner at a relatively low ratio value of starting PCC particles relative to the number of milling media particles (i.e., the xe2x80x9cP/M particle ratioxe2x80x9d) present in the mill grinding runs and while free or essentially free of the presence of organic dispersant so as to achieve a significantly enhanced (narrower) particle size distribution (i.e., reduced slope) as compared to PCC particles subjected to comminution in a single, high energy milling pass and/or as done in high solids content (e.g,  greater than 50 wt % solids) environments in the presence of an organic dispersant. The use of multi-stage grinding serves to compensate for the longer residence time otherwise associated with milling at the aforesaid low ratio value of PCC particles/media particles according to this embodiment of the invention. The aforesaid milling conditions of this embodiment of the invention provide a comminution in which agglomerates of primary PCC particles that typically arise during the prior synthesis of the PCC are effectively broken-up, but without grinding the liberated primary particles to form a quantity of fines and ultrafines that would undesirably broaden the particle size distribution. The narrower particle size distribution achieved by this embodiment of the invention translates into improved brightness and opacity effects for paper coating applications because of the less efficient pigment particle packing that is obtained given the general absence of fine particles. The terminology xe2x80x9cessentially freexe2x80x9d means that if any organic dispersing agent is present at the time of the grinding that it is present only in such scant benign amounts that the 75/25 slope value, defined herein, can still be lowered below 1.8 by the inventive processing. In no event is any amount of organic dispersant present during the wet media milling step that is equal or greater than 0.01 wt % based on the dry weight of calcium carbonate in the slurry at that point. The median particle size of the PCC particles of the high solids content PCC slurries obtained by processing according to this invention generally will be in range of approximately 0.25 to 2.0 xcexcm, and more typically between approximately 0.5 to 1.0 xcexcm, and within a relatively narrow size distribution as defined by a slope (steepness factor) of less than approximately 1.8, and more preferably less than approximately 1.75. The xe2x80x9cslopexe2x80x9d, as that term is used in this invention, means the quotient value of the diameter value for which 75% of the particles are less than (as the numerator), divided by the diameter value for which 25% of the particles are less than (as the denominator), where the particle sizes are measured by a Sedigraph Particle Size Analyzer. Indeed, even smaller slope values between 1.6 to 1.7 are routinely achievable by the present invention. The PCC slurries fed to the inventive multi-pass wet media grinding procedure can be freshly made precipitated PCC slurries, or non-treated (i.e., non-dispersed, non-comminuted, non-dewatered) raw PCC slurries that have been previously made, stored and supplied, such as applicable commercially available coarse grade PCC slurries.
In another aspect of the invention, it has been discovered that the PCC slurries that are subjected to the solid bowl centrifuging step in a dispersed state according to another step of the invention achieve such extensive dewatering that the resulting high solids content of the partly dewatered PCC slurry (viz., up to about 75 wt % solids) permits reductions in demands for, if not permitting complete elimination of, separate cost-intensive evaporative dehydration procedures in the process flow.
In yet another aspect of the invention, it has been discovered that high solids content PCC slurries, such as those that are greater than 70% solids by weight, can be subjected to a high shear in-line mixing step performed using a rotor and stator mixer so as to impart a reduced high shear viscosity property, yet without sacrificing favorable particle distribution and median particle size properties of the treated PCC particles (such as imparted in a preceding multi-stage milling step per another aspect of this invention). For example, a high solids content PCC slurry subjected to the high shear, in-line mixing via a rotor and stator mixer according to this step of the inventive process can achieve a Hercules shear viscosity of 18+ dynes at 935 rpm (A-bob @ 100 rpm) indicative of high shear flowability.
Consequently, the aqueous PCC slurries prepared by this invention are quite useful for paper coating applications as they yield a coating color for paper that has excellent high-shear flowability under an applicator blade to permit uniform coatings to be applied to fast running paper webs, yet have reduced mobility after being coated such that it will not run or wick. A fine particle size precipitated calcium carbonate is provided (i.e., the median particle size  less than 2 microns) within a narrow particle size distribution (i.e., at a slope  less than 1.8). The high solids PCC slurries of this invention show significantly reduced dilatant rheological behavior as compared to PCC coating pigment slurries processed by conventional means. Moreover, the PCC slurries of this invention and paper coating compositions incorporating same have been observed to be rheologically stable over time.
PCC slurries comprised of calcitic crystal structures of PCC, and preferably rhombohedral shaped particle form, are preferably, although not exclusively, processed by this invention to provide high quality PCC slurries well-suited for use in paper coating applications. To best ensure the rheological stability, the PCC particles that are processed according to the invention preferably have a rhombohedral shaped calcitic crystal form.